I am experience a perplexing phenomenon. My pool is 16X32, 3.5' to 7.5' deep vinyl. I am not losing water unusually. I am getting a noticeable amount of what appears to be dirt after filter has run. I have a Fury Robot that vacuums the pool excellently. I have not found any tears or leaks. There is no filter sand mixed with the dirt. Any ideas what the cause might be? Thanks